1. Field of the Invention
The present invention relates to metal powders having a narrow particle size distribution for use in metallic pigments. The present invention also relates to a process for classifying fine metal powders.
2. Description of Prior Art
Various metal powders are used in many fields such as powder metallurgy, pigments, reducing agents, catalysts and others. Among them, graded metal powders having relatively larger particle sizes, such as those for powder metallurgy, reducing agents or pigments, have heretofore been produced by classifying the metal powders having a broad particle size distribution which had been produced by grinding metals or atomizing molten metals using a sieve or an air classifier. In particular, in order to obtain an ornamental effect of metallic feeling or touch, fine metal powders for use in metallic pigments have been produced by selecting finer metal powder portion from the graded metal powder product, followed by expanding and grinding it using a ball mill or a stamp mill to form small flakes having a thickness of 0.2 to 2 .mu.m and a surface area of 10 to 10,000 .mu.m.sup.2 in total of the opposite surfaces.
As stated above, the graded metal powders have conventionally been produced by classifying metal particles using sieves or by air stream classifiers. More specifically, the sieves used in industry have interstices of at least 30 .mu.m, and therefore classification of metal particles having particle sizes smaller than 30 .mu.m has heretofore been performed using air stream classifiers. However, the particle size distribution of metal powders obtained by air stream classification has been relatively broad. For this reason, when it is desired to produce finer metal powders for metallic pigments, it is necessary to strictly control the conditions of production, particularly those for expanding and grinding metal particles using a ball mill or a stamp mill, and heretofore, it has been very often the case that the quality of the fine metal powders for metallic pigments is not always uniform since metal particles are not always the same in size and in the chance of collision with balls in the ball mill or stamp mill or in the amount of force per unit surface area received by the particles.
For example, when metal powder which includes two kinds of particles, one having a diameter of 30 .mu.m and another having a diameter of 2 .mu.m, is ground using a ball mill, the force per unit area of cross-section urged on the particles having a diameter of 2 .mu.m is 225 times as great as the force per unit area of cross-section urged on the particles having a diameter of 30 .mu.m, assuming that the balls in the ball mill have the same weight and collide with the two kinds of particles at the same velocity. In other words, the smaller particles can be expanded to form thin, small flakes with a smaller number of pinching actions by the balls, while the larger particles need a larger number of pinching actions for forming similar thin, small flakes, and during the grinding step, the grinding of the flakes derived from the smaller particles precedes, thus forming many ultrafine particles. Some of the ultrafine particles tend to be driven in the surfaces of the larger particles during their expansion to generate a number of flaws thereon. When a paint or coating material is prepared from the fine metal powder composed of the relatively large flakes having many flaws on their surface and many ultrafine particles as a pigment, and the resulting paint is coated on substrates such as a glass plate, the surfaces of the relatively large particles reflect light irregularly, the ultrafine particles dispersed in the coating layer also reflecting light irregularly, with the result that the coating layer fails to give a good luster or metallic feeling or touch.
Conventional processes for the classification of powders having a particle size not greater than 30 .mu.m which do not react with water or air such as powders of oxides include a process in which the powders are dispersed in water and classified making use of the difference in the sedimentation rate in water of the particles in the powder according to their size. However, this process is not applicable to the classification of fine powders of metals, in particular, active metals such as magnesium, aluminum and titanium, because the powders of such metals are oxidized at high rates in water containing air, with the result that not only the chemical quality of the graded powder deteriorates but also the classification itself is prevented, thus failing to provide graded particles.